As disclosed in United Kingdom patent specification No. 908,609, attempts have been made at converting urea into cyanic acid by pumping molten urea through a strongly heated tube, presumably to evaporate the urea. These experiments were not successful in developing a technically suitable process, since cyanuric acid which is a by-product of the process blocked the heated tube. It is believed that the by-product cyanuric acid is produced concomitantly with the initial decomposition of urea into cyanic acid and ammonia. The cyanuric acid deposition on the walls of the tubular reactor and results in ultimately blocking the heated tubular reactor. The layer of cyanuric acid impedes the transfer of heat from the tube wall to the molten urea, so that no more evaporation of urea takes place. Rather the molten urea is only heated. The fact that urea is only heated, rather than evaporated, leads to the formation of more cyanuric acid, which also deposits on the wall.
The formation and depositing of solid by-products can be prevented by evaporating urea in a finely divided state by direct heat exchange with a heated gas mixture obtained by converting cyanic acid formed by thermal cracking of urea into melamine and separating this melamine from the remaining cracking gases, in a manner described in U.S. Pat. No. 3,300,492. In this process the cracking of the urea may be effected in a vertical column. Optionally the heat required may be supplied through the wall of the column. A drawback of the process according to U.S. Pat. No. 3,300,492 is that a separate heater is needed to heat the recycled gas mixture to the temperature required for cracking.